Voltage converter systems

ABSTRACT

A voltage converter system includes a first DC-AC voltage converter that converts a first DC voltage signal to a first AC voltage signal. A DC link converts the first AC voltage signal to a second DC voltage signal. A second DC-AC voltage converter converts the second DC voltage signal to a second AC voltage signal. In another configuration a DC-AC voltage converter converts a DC voltage signal to a first AC voltage signal. An AC-AC voltage converter converts the first AC voltage signal to a second, lower-frequency AC voltage signal. In yet another configuration a first voltage converter portion converts a DC voltage signal to pulses of DC voltage. A second voltage converter portion converts the pulses of DC voltage to a relatively low-frequency AC voltage signal. The voltage converter system is selectably configurable as a DC-AC voltage converter or an AC-DC voltage converter.

This application claims priority to U.S. provisional application No.61/785,958, filed Mar. 14, 2013, the contents of which are herebyincorporated by reference.

FIELD

The present invention relates generally to voltage converter systems, inparticular to systems adapted to convert direct-current (DC) voltages toalternating-current (AC) voltages and vice versa.

BACKGROUND

A DC-AC voltage converter is an electrical system that changes a DCvoltage to an AC voltage. The converted AC voltage may have any desiredvoltage level, waveform and frequency with the use of appropriatetransformers, switching, filtering and control circuits. DC-AC voltageconverters are used in a wide range of applications, from smallswitching power supplies in electronic devices such as computers tolarge electric utility high-voltage direct current applications thattransport bulk power. DC-AC voltage converters are also commonly used tosupply AC power from DC sources such as solar panels or batteries.

FIG. 1 shows a typical prior art DC-AC voltage converter 10, whichoperates at a relatively low frequency. Voltage converter 10 isrelatively simple, but it suffers from significant disadvantages. Afirst disadvantage is cost, because it uses a low-frequency transformer12 that requires a relatively large amount of copper for transformerwindings. In recent years the cost of copper has increased, while thecost of power semiconductors has decreased. This trend is expected tocontinue. In addition, a low-frequency transformer has relatively lowefficiency when it is configured with a relatively high winding turnsratio and is used for voltage step-up. An example of such configurationsis a DC-AC voltage converter with a step-up transformer having a turnsratio of about 19:1 or more and a relatively low input voltage powersource, for example about 10 to 20 volts DC.

SUMMARY

Given the foregoing, it is desirable to perform voltage conversion witha relatively high-frequency transformer driven by suitable powerswitching semiconductors. In one embodiment the present invention is aDC-AC voltage converter capable of operating with a relatively low DCvoltage source input, such as from a battery power supply.

In some embodiments of the present invention the DC-AC voltage convertermay be bidirectional, thereby capable of receiving an AC voltage signaland generating an output DC voltage signal. This arrangement is useful,for example, for charging a battery from an AC grid.

Preferably, a transformer is utilized to provide electrical isolationfor DC-AC and AC-DC conversion. For example, an isolation transformermay be used between a DC voltage input (e.g., a battery) and an ACvoltage output. The voltage converters of the present invention may begenerally divided into several types according to the type oftransformer selected. For example, the isolation transformers may berelatively low-frequency, on the order of 50/60 Hertz (Hz). Preferably,the isolation transformers are relatively high-frequency, on the orderof tens or more kilohertz (kHz).

An aspect of the present invention is a voltage converter system thatincludes a first, high-frequency, DC-AC voltage converter configured toreceive a first DC voltage signal and generate a first AC voltagesignal. A DC link is configured to receive the first AC voltage signaland convert the first AC voltage signal to a second DC voltage signal. Asecond DC-AC voltage converter is configured to receive the second DCvoltage signal and generate a second AC voltage signal.

Another aspect of the present invention is a voltage converter systemthat includes a DC-AC voltage converter configured to receive a DCvoltage signal and generate a first, relatively high-frequency, ACvoltage signal. An AC-AC voltage converter is configured to receive thefirst AC voltage signal and generate a second AC voltage signal. Thefrequency of the second AC voltage signal is preferably lower than thefrequency of the first AC voltage signal.

Yet another aspect of the present invention is a voltage convertersystem that includes a first voltage converter portion that isconfigured to receive a DC voltage signal and convert the DC voltagesignal to pulses of DC voltage. A second voltage converter portion isconfigured to receive the pulses of DC voltage and convert the pulses ofDC voltage to a relatively low-frequency AC voltage signal. The voltageconverter system is selectably configurable as a DC-AC voltage converteror an AC-DC voltage converter. In some embodiments of the presentinvention the first voltage converter portion includes a Ćuk-typevoltage converter and a single-ended primary inductor converter (SEPIC)voltage converter, the Ćuk-type voltage converter and the SEPIC voltageconverter being electrically combined to operate cooperatively.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the inventive embodiments will become apparent tothose skilled in the art to which the embodiments relate from readingthe specification and claims with reference to the accompanyingdrawings, in which:

FIG. 1 is an electrical schematic diagram of a typical prior art DC-ACvoltage converter;

FIG. 2 is an electrical schematic diagram of a DC-AC voltage convertersystem with a DC link according to an embodiment of the presentinvention;

FIG. 3 is an electrical schematic diagram of a DC-AC voltage convertersystem without a DC link according to another embodiment of the presentinvention;

FIG. 4 is an electrical schematic diagram of a voltage converterconfigurable for operation as either a DC-AC or an AC-DC voltageconverter according to yet another embodiment of the present invention;

FIG. 5 is an electrical schematic diagram showing details of a firstportion of the voltage converter of FIG. 4;

FIG. 6 is a graph showing the general waveform of certain electricalsignals generated by the circuit of FIG. 5;

FIG. 7 is an electrical schematic diagram showing details of a secondportion of the voltage converter of FIG. 4;

FIG. 8 is an electrical schematic diagram of a Ćuk-type voltageconverter;

FIG. 9 is an electrical schematic diagram of a single-ended primaryinductor converter voltage converter;

FIG. 10 is an electrical schematic diagram of the voltage converters ofFIGS. 8 and 9 electrically combined together in a new arrangement inaccordance with an embodiment of the present invention, providing for areduced total component count;

FIG. 11 is an electrical schematic diagram of the voltage converter ofFIG. 10 incorporating several refinements; and

FIG. 12 is an electrical schematic diagram of a voltage converteraccording to yet another embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 2 shows a DC-AC voltage converter system 100 having a first,high-frequency, DC-AC voltage converter 102 according to an embodimentof the present invention. First DC-AC voltage converter 102 receives atan input 103 a first DC voltage signal. A first, relativelyhigh-frequency, AC voltage signal 104 generated by a transformer 105 offirst DC-AC voltage converter 102 is supplied to a DC-link 106 thatconverts the first AC voltage signal to a second DC voltage signal 108.Second DC voltage signal 108 is coupled to a second DC-AC voltageconverter 110 that converts second DC voltage signal 108 to a second ACvoltage signal, output AC voltage signal 112. Output 112 may have eitherlow-frequency components, high-frequency components, or both low- andhigh-frequency components.

An optional electrical filter 114 provides filtering of AC outputvoltage signal 112 to remove high-frequency components and/or limitelectromagnetic interference (EMI) caused by the AC output voltagesignal, resulting in a filtered AC output voltage signal 116. Forcertain applications where power quality is not a significant issue(such as a motor drive, as one example) a filter 114 configured toremove high-frequency components may be omitted.

FIG. 3 shows a DC-AC voltage converter system 200 according to anotherembodiment of the present invention. A first AC voltage signal 202generated by a DC-AC voltage converter 204 is supplied to an AC-ACvoltage converter 206 that converts the first AC voltage signal to asecond AC voltage signal, output AC voltage signal 208. An electricalfilter 210 provides filtering of AC output voltage signal 208 to reduceEMI caused by the AC output voltage signal, resulting in a filtered ACoutput voltage signal 211. First AC voltage signal 202 is a relativelyhigh-frequency voltage signal, while second AC voltage signal 208 is arelatively low-frequency voltage signal output from voltage convertersystem 200.

With reference to FIGS. 2 and 3 together, voltage converter system 100provides relatively efficient voltage conversion, but compared tovoltage converter system 200 it is more complex and more expensive toproduce. However, the performance of voltage converter system 200depends in part upon the operating conditions of a transformer 212. FIG.3 shows a topology wherein transformer 212 operates under regulation,with a relatively high turns ratio. In this case efficiency of voltageconverter system 200 will be less and the voltage converter system willgenerate a relatively high level of EMI on the AC output voltage signal208. Consequently, EMI filter 210 may require a number of relativelyexpensive components in order to be effective.

FIG. 4 shows a schematic diagram of a voltage converter system 300according to yet another embodiment of the present invention. Voltageconverter system 300 is configurable for operation as either a DC-AC oran AC-DC voltage converter and is suitable for low DC input voltages(e.g., on the order of about 8-16 VDC) at power levels of up to severalkilowatts. Furthermore, voltage converter system 300 overcomes thedisadvantages discussed above. Voltage converter system 300 may beimplemented with a relatively low number of active semiconductorswitches. In addition, a transformer 302 (comprising windings 302A,302B) functions under extremely benign conditions (i.e., conditionsfavorable in that root-mean-square (RMS) current and RMS voltage arefavorable for relatively low transformer losses). Finally, there is onlya low level of EMI on the AC side.

The topology of voltage converter system 300 may be divided into twoportions for the purpose of explanation. A first voltage converterportion, 400 shown in FIG. 5, provides pulses of DC voltage regulatedfrom 0 volts to a predetermined maximum voltage, with a generallyhalf-sinusoidal waveform as shown in FIG. 6. A second voltage converterportion 500, shown in FIG. 7, provides electrical isolation andconversion from the pulsed DC voltage of FIG. 6 to a predeterminedrelatively low-frequency AC voltage signal including, withoutlimitation, about 120 VAC at a frequency of about 50/60 Hz.

With continued reference to FIG. 5, this power stage is a combination oftwo types of power converters. The first is a Ćuk-type voltage converter600, shown in FIG. 8. The other is a single-ended primary inductorconverter (SEPIC) voltage converter 700, shown in FIG. 9. Theoperational details of these voltage converters are well-known in theart and thus will not be further elaborated upon here. Both voltageconverters have a number of common features. For example, each iscapable of providing an output voltage from zero to several times higherthan the input voltage. In addition, both are bi-directional.

One important difference between the Ćuk-type voltage converter and theSEPIC-type voltage converter is that the Ćuk-type voltage converterreverses the polarity of the input voltage while the SEPIC-type voltageconverter does not. With reference again to FIG. 5, thesecharacteristics may be utilized to advantage, to provide an outputvoltage from an appropriately paired and electrically combined Ćuk-typevoltage converter and SEPIC-type voltage converter that is about twicethe output voltage available from each voltage converter individually,each voltage converter providing about half of output power delivered bythe electrically combined voltage converters. A further advantage ofthis arrangement is that doubling the output voltage in this manner aidsto reduce the required primary-to-secondary winding turns ratio ofisolation transformer 302.

With reference now to FIGS. 8 and 9 together, switches 602, 702respectively exhibit substantially the same operating characteristics.Likewise, inductors 604, 704 in FIGS. 8 and 9 respectively exhibitsubstantially the same operating characteristics. Therefore, thesecomponents can be combined in an appropriately paired Ćuk-type voltageconverter and SEPIC-type voltage converter to form the circuit 800 shownin FIG. 10. In FIG. 10, switch 802 replaces switches 602, 702 whileinductor 804 replaces the inductors 604, 704. Thus, switch 802 andinductor 804 are common to both the Ćuk-type voltage converter and theSEPIC voltage converter. This results in one less active switch and oneless inductor in an appropriately paired Ćuk-type voltage converter andSEPIC-type voltage converter, thereby reducing voltage converter cost.Circuit 800 may be substituted for circuit 400 in the system of FIG. 4.

Ćuk and SEPIC voltage converters have one common disadvantage in thatneither provide forward power conversion. Rather, they use passivecomponents such as capacitors and inductors for energy storage.Consequently, the efficiency of these voltage converters depends verymuch on the quality factor of the aforementioned passive components. Thequality factor of capacitors are generally good, but the quality factorof inductors are often less than desirable and often tend to worsenunder high-current and low-voltage operating conditions. To reducelosses and increase efficiency, system 800 may be modified, replacinginductor 804 with an inductor/transformer 904, as shown in the circuit900 of FIG. 11. In this embodiment of the present invention when aswitch 902 begins conducting forward power conversion will be providedby inductor/transformer 904, thereby increasing the efficiency of system900 in comparison to system 800 of FIG. 10. Circuit 900 may besubstituted for circuit 400 in the system of FIG. 4.

With reference again to FIG. 7, voltage converter portion 500 comprisesa power stage which will provide isolation between the low voltage sideand the high voltage side. This topology is a series-resonant voltageconverter, which is bi-directional. The power transformer 302 in thiscase works under substantially benign conditions, with a generallytrapezoidal voltage wave form and a generally sinusoidal current waveform. The transformer 302 leakage inductance is part of the resonantinductor or, optionally, may comprise the entire resonant inductor. Allthese features aid to keep efficiency and the commutation frequency ashigh as possible. This reduces the transformer size and reduces itscost, as well as total inverter cost, reducing the cost of EMI filtersif used.

A voltage converter 1000 is shown in FIG. 12 according to yet anotherembodiment of the present invention. Like voltage converters 800 and900, voltage converter 1000 may be substituted for circuit 400 in thesystem of FIG. 4.

Voltage converter 1000 includes a first inductor 1002 and a secondinductor 1004 connected in series, the first and second inductors eachhaving an input and an output. A first capacitor 1006 is electricallyintermediate the first and second inductors 1002, 1004, a first terminalof the first capacitor being electrically connected to the output of thefirst inductor and a second terminal of the first capacitor beingelectrically connected to the input of the second inductor. A thirdinductor 1008 and a fourth inductor 1010 are connected in series, thethird and fourth inductors each having an input and an output. A secondcapacitor 1012 is electrically intermediate the third and fourthinductors 1008, 1010, a first terminal of the second capacitor beingelectrically connected to the output of the third inductor and a secondterminal of the second capacitor being electrically connected to theinput of the fourth inductor. A first switch 1014 is coupled between theinput of the first inductor 1002 and the output of the third inductor1008. A second switch 1016 is coupled between the output of the firstinductor 1002 and the input of the third inductor 1008. A rectifier 1018is arranged such that an anode of the rectifier is electricallyconnected to the second terminal of the first capacitor 1006, a cathodeof the rectifier being electrically coupled to the second terminal ofthe second capacitor 1012. A third switch 1020 is electrically connectedin parallel with the rectifier 1018. Voltage converter 1000 isconfigured to receive a DC voltage signal at the inputs of the first andthird inductors 1002, 1008 and to generate an AC voltage signal at theoutputs of the second and fourth inductors 1004, 1010.

Voltage converter system 1000 may further include third capacitor 1022,the third capacitor being electrically intermediate the second andfourth inductors 1004, 1010. A first terminal of third capacitor 1022 iselectrically connected to the output of the second inductor 1004 and asecond terminal of the third capacitor is electrically connected to theoutput of the fourth inductor 1010.

The foregoing configuration of voltage converter system 1000 has theadvantage of relatively low inductor current and a low switch current,similar to the embodiment of FIG. 5, since there are two input inductors(1002 and 1008) rather than the single input inductor of thepreviously-described configurations, and also has a low number ofswitches similar to the embodiment of FIG. 10. It should be noted thatvoltage converter system 1000 has more input current ripple compared tothe embodiment of FIG. 5, as half of the input current is discontinuousbecause it flows through the switches, it is important in thisembodiment that the switches switch synchronously to eliminate voltagetransients across the switches and losses.

Inductors 1002, 1008 of voltage converter system 1000 may optionally becoupled magnetically to allow current balancing to occur. The current ininductor 1008 and switch 1014, and in inductor 1002 and switch 1016, maynot necessarily ramp up identically as these inductor-switch pairs areindependent of one another. However, when switches 1014, 1016 are openedthe current flows in a complete circuit through the output (i.e., “a”and “b” of FIG. 12) so the current in inductors 1002, 1008 must besubstantially the same. Any error will result in the energy being dumpedin the switches 1014, 1016 until the currents are substantially thesame. If the windings 1002, 1008 are coupled the energy can transferbetween the windings until the currents are substantially the samerather than the energy being lost.

In some embodiments of the present invention certain inductors ofvoltage converter system 1000 may be wound upon a common core. Forexample, inductors 1002, 1008 may be wound upon a common core.Similarly, inductors 1004, 1010 may be wound upon a common core. Windingthe inductors upon a common core may provide certain advantages, such asa reduction in the overall size of the inductors.

One skilled in the art will appreciate that any suitable electroniccomponents may be utilized for the circuits shown in the accompanyingfigures and described herein. For example, the switches may be anysuitable types of power switching components including, withoutlimitation, semiconductors such as bipolar junction transistors, fieldeffect transistors and thyristors. Likewise, the diodes, capacitors,inductors and transformers shown in the accompanying figures may be anysuitable types and values for a particular realization of the circuitry.

In addition, the circuits shown in the accompanying figures aresimplified for purposes of explanation and are not intended to belimiting in any way. Accordingly, the circuits may include any suitablenumber and type of ancillary components including, without limitation,biasing, feedback and filtering components and circuitry as well asanalog and/or digital monitoring, feedback and control circuitry.

While this invention has been shown and described with respect to adetailed embodiment thereof, it will be understood by those skilled inthe art that changes in form and detail thereof may be made withoutdeparting from the scope of the claims of the invention.

What is claimed is:
 1. A voltage converter system, comprising: a first,high-frequency, DC-AC voltage converter configured to receive a first DCvoltage signal and generate a first AC voltage signal; a DC linkconfigured to receive the first AC voltage signal and convert the firstAC voltage signal to a second DC voltage signal; and a second DC-ACvoltage converter configured to receive the second DC voltage signal andgenerate a second AC voltage signal.
 2. The voltage converter system ofclaim 1, further including an electrical filter configured to limitelectromagnetic interference generated by the second AC voltage signal.3. A voltage converter system, comprising: a DC-AC voltage converterconfigured to receive a DC voltage signal and generate a first,relatively high-frequency, AC voltage signal; and an AC-AC voltageconverter configured to receive the first AC voltage signal and generatea second AC voltage signal, a frequency of the second AC voltage signalbeing lower than the frequency of the first AC voltage signal.
 4. Thevoltage converter system of claim 3, further including an electricalfilter configured to limit electromagnetic interference generated by thesecond AC voltage signal.
 5. The voltage converter system of claim 3wherein the DC-AC voltage converter includes a transformer having arelatively high turns ratio, the transformer further operating underregulation.
 6. A voltage converter system, comprising: a first voltageconverter portion, configured to receive a DC voltage signal and convertthe DC voltage signal to pulses of DC voltage; and a second voltageconverter portion, configured to receive the pulses of DC voltage andconvert the pulses of DC voltage to an AC voltage signal.
 7. The voltageconverter system of claim 6 wherein the pulses of DC voltage arehalf-sinusoidal pulses.
 8. The voltage converter system of claim 6wherein the first voltage converter portion includes: a Ćuk-type voltageconverter; and a single-ended primary inductor converter (SEPIC) voltageconverter, the Ćuk-type voltage converter and the SEPIC voltageconverter being electrically combined to operate cooperatively.
 9. Thevoltage converter system of claim 8 wherein the pulses of DC voltageproduced by the electrically combined Ćuk-type voltage converter andSEPIC voltage converter have an output voltage about twice the voltageavailable from each converter individually, each voltage converterproviding about half of output power delivered by the voltage convertersystem.
 10. The voltage converter system of claim 8 wherein a switch andan inductor are common to both the Ćuk-type voltage converter and theSEPIC voltage converter.
 11. The voltage converter system of claim 8wherein the first voltage converter portion includes aninductor/transformer.
 12. The voltage converter system of claim 6wherein the first voltage converter portion includes: a first and asecond inductor connected in series, the first and second inductors eachhaving an input and an output; a first capacitor electricallyintermediate the first and second inductors, a first terminal of thefirst capacitor being electrically connected to the output of the firstinductor and a second terminal of the first capacitor being electricallyconnected to the input of the second inductor; a third and a fourthinductor connected in series, the third and fourth inductors each havingan input and an output; a second capacitor electrically intermediate thethird and fourth inductors, a first terminal of the second capacitorbeing electrically connected to the output of the third inductor and asecond terminal of the second capacitor being electrically connected tothe input of the fourth inductor; a first switch coupled between theinput of the first inductor and the output of the third inductor; asecond switch coupled between the output of the first inductor and theinput of the third inductor; a rectifier having an anode and a cathode,the anode of the rectifier being electrically connected to the secondterminal of the first capacitor and the cathode of the rectifier beingelectrically coupled to the second terminal of the second capacitor; anda third switch electrically connected in parallel with the rectifier,the voltage converter system being configured to receive a DC voltagesignal at the inputs of the first and third inductors and to generate anAC voltage signal at the outputs of the second and fourth inductors. 13.The voltage converter system of claim 12, further including a thirdcapacitor, the third capacitor being electrically intermediate thesecond and fourth inductors, a first terminal of the third capacitorbeing electrically connected to the output of the second inductor and asecond terminal of the third capacitor being electrically connected tothe output of the fourth inductor.
 14. The voltage converter system ofclaim 12 wherein the switches switch synchronously.
 15. The voltageconverter system of claim 14 wherein the switches are semiconductors.16. The voltage converter system of claim 12 wherein at least twoinductors are magnetically coupled.
 17. The voltage converter system ofclaim 6 wherein the voltage converter system is selectably configurableas a DC-AC voltage converter and an AC-DC voltage converter.
 18. Thevoltage converter system of claim 6 wherein the second voltage converterportion further provides electrical isolation.
 19. The voltage convertersystem of claim 6 wherein the AC voltage signal has a relatively lowfrequency.
 20. The voltage converter system of claim 19 wherein the ACvoltage signal has a voltage of about 120 VAC and a frequency of about60 Hz.